This disclosure is generally related to electrochromic devices, electrodes therefor, and methods of forming the electrodes.
An electrochromic device is a self-contained, two-electrode (or more) electrolytic cell that includes an electrolyte and one or more electrochromic materials. Electrochromic materials can be organic or inorganic, and reversibly change visible color when oxidized or reduced in response to an applied electrical potential. Electrochromic devices are therefore constructed so as to modulate incident electromagnetic radiation via transmission, absorption, or reflection of the light upon the application of an electric field across the electrodes. The electrodes and electrochromic materials used in the devices are dependent on the type of device, i.e., absorptive/transmissive or absorptive/reflective.
Absorptive/transmissive electrochromic devices typically operate by reversibly switching the electrochromic materials between colored and bleached (colorless) states. Typical electrochromic materials used in these devices include indium-doped tin oxide (ITO), fluorine-doped tin oxide (SnO2:F), poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT-P SS), and single-walled carbon nanotubes (SWNT). Flexible electrochromic devices of this type have been constructed using a flexible substrate layer of polyethylene terephthalate (PET), a transparent layer of ITO as the working electrode, and a third layer of PEDOT-PSS as the counter electrode. Drawbacks with ITO include high cost and lower flexibility compared to PEDOT-PSS. PEDOT-PSS, however, suffers from a lower conductivity value compared to ITO. A further drawback of ITO devices for some applications is that in order for light to pass through the device, the electrodes must be transparent.
The absorptive/reflective-type electrochromic devices typically contain a reflective metal as an electrode. The electrochromic polymer is deposited onto this electrode and is faced outward to allow incident light to reflect off the polymer/electrode surface. The counter electrode is behind the active electrode. Similar electrode and electrochromic materials can be used in these reflective devices, in particular ITO and PEDOT-PSS.
This disclosure addresses the ongoing challenge of making flexible electrochromic devices without ITO that exhibit one or more advantageous properties, such as fast switching, high contrast, flexibility, and in some cases stretchability. It would be a further advantage if such devices did not require a transparent electrode or transparent conductive substrate.